Fiber coupled optical polarization beam splitters and combiners are used in fiber-based communication systems to perform various functions, for example to combine two laser beams having orthogonal polarizations to produce a single beam having a higher total output power as well as low polarization dependence. Splitting a beam is fairly straightforward but combining beams is a more difficult proposition, at least when the goal is to combine beams without excessive losses. Generally, beams can only be combined without excessive losses where the two beams being combined have different wavelengths, or when the two beams being combined have orthogonal polarizations. If the two beams have aligned polarization, the resulting interference creates a substantial instability of optical power.
A couple of approaches have been tried in the prior art to split and combine beams. FIG. 1 illustrates beam splitter 100 known in the prior art. The beam splitter 100 comprises a housing 102 within which is placed a polarizing beam splitter 104. Three collimators 106, 108 and 110 are attached to the housing 102 and each is coupled to an optical fiber; generally, the collimator 106 will be coupled to a single-mode (SM) optical fiber, while the collimators 108 and 110 will be connected to polarization-maintaining (PM) fibers. When the beam splitter 100 operates as a splitter, a light beam enters through the fiber 112 and the collimator 106, is split by the polarizing beam splitter 104, and the resulting beams are output through the collimators 108 and 110 to the optical fibers 114 and 116, respectively. The beam splitter 100 can operate as a combiner, but only if the inputs are different polarizations. Thus, when the beam splitter 100 operates as a combiner, two polarized beams are input through the fibers 114 and 116 and are combined into a single beam by the polarizing beam splitter 104, and are output through the collimator 106 into the fiber 112. The beam splitter 100, however, suffers from various disadvantages. First, it is big and bulky, making it difficult to integrate with optical packages, which tend to be very small. Second, it requires three collimators that must be very precisely aligned with the polarizing beam splitter for the device to work properly. Finally, even when the device 100 works properly is has a low extinction ratio.
FIG. 2 illustrates another beam splitter and combiner 200 known in the prior art. The beam splitter/combiner 200 comprises a pair of polarization beam displacer wedge pairs 204 sandwiched between a pair of collimators 202. Each collimator 202 comprises a grin lens 208 coupled to a ferrule 206; the ferrule 206 will either be a single-fiber ferrule of a two-fiber ferrule, as the case may be. The beam splitter/combiner 200 has several disadvantages. The large number of parts makes the splitter/combiner more expensive. Moreover, the large number of parts means that the splitter/combiner is more difficult to manufacture accurately, for example due to the tolerance buildup involved in assembling so many parts. Finally, the polarization beam displacer wedge pairs 204 create more losses and must be aligned very accurately.